There has been conventionally utilized rainwater from which sand and fine dusts are eliminated and which is stored. And a rainwater filtration device for eliminating sand and dusts from the rainwater has been proposed (See Patent Document 1, for example). This filtration device comprises an outer pipe and an inner pipe provided inside this outer pipe; a vertical portion is formed in the outer pipe; and the upper end of the outer pipe is connected to a rain gutter. The lower end of the outer pipe is connected to a catch basin communicating with a sewer culvert. An outer peripheral edge of the upper end of the inner pipe is arranged with a predetermined interval from an inner peripheral surface of the vertical portion. The lower end of the inner pipe is connected to a rainwater storage tank. A part of the vertical portion of the outer pipe is formed so as to continuously change its inner diameter, and an upper part of the inner pipe is configured so as to enable adjustment of the predetermined interval by moving the inner pipe in the vertical direction. An opening portion is provided at the outer pipe in the vicinity of the upper end of the inner pipe, and a lid, which can open/close the opening portion is provided at this opening portion.
In this filtration device, rainwater flowing down through the rain gutter flows down in the layered state while adhering to the inner peripheral surface of the vertical portion of the outer pipe. At a small- to medium-scale rainfall when the rainfall intensity does not exceed the flowing-down capacity of the sewer culvert and rivers, the thickness of the layer of the rainwater flowing down on the inner peripheral surface of the vertical portion is smaller than the interval between the outer peripheral edge of the upper end of the inner pipe and the inner peripheral surface of the vertical portion, and thus the rainwater flows down only through the outer pipe into the sewer culvert via the catch basin but does not flow through the inner pipe.
At a large-scale rainfall when the rainfall intensity exceeds the flowing-down capacity of the sewer culvert and the like, the thickness of the layer of the rainwater flowing down on the inner peripheral surface of the vertical portion becomes larger than the interval, and thus the rainwater also flows down the inner pipe into the rainwater storage tank. As a result, this filtration device can divide and lead the rainwater from the rain gutter into the storage tank only at the rainfall intensity exceeding the flowing-down capacity of the sewer culvert and rivers, can be installed in a small space of an existing rain gutter with a simple structure, and can eliminate sand and fine dusts entering from the rain gutter and lead the rainwater to the storage tank.
Patent Document 1: Japanese Patent Laid-Open No. 64-83738 (Claims, FIG. 1)